13. Mapping of the ORGANLESS1 gene, ORL1

1)Bioscience Center, Nagoya University, Nagoya, 464-8601 Japan
2)Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, 113-8657 Japan
3)Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan

During embryogenesis in rice, the shoot apical meristem (SAM) and root apical meristem (RAM) are established based on the basic body plan of an embryo. After germination, most morphogenetic events depend on the activity of these apical meristems. Therefore, the mechanism of the meristem establishment during embryogenesis should be one of the most important subjects in developmental plant biology. To elucidate the mechanism of the SAM and RAM establishment, we are trying to isolate the ORGANLESS1 gene, ORL1 by positional cloning. The organless mutant has been isolated and briefly characterized previously (Hong et al 1995, Sato et al. 1996). This mutant fails to develop most organs and tissues such as SAM, RAM

and vascular tissues, except the scutelum (Fig. 1A). In the orl1 embryo, expression of a rice homeobox gene, OSH1 which is specifically expressed in the shoot apical region before morphological development, is similar to that in the wild-type embryo (Fig. 1B). This observation implies that pattern formation may be normally accomplished, but the following organ differentiation would be impaired in the orl1 embryo.

Because the orl1 embryo develops no shoot or root, we cannot obtain enough genomic DNA for mapping. Therefore, we used the F2 ORL1 heterozygous plants. The ORL1 genotypes in F2 plants were identified by screening the phenotypes of F3 embryos. We used 50 F2 plants with heterozygous orl1 alleles for this linkage analysis. The linkage analysis has demonstrated that ORL1 is located between S10628 and C377, on chromosome 4 (Fig. 2). According to the RGP (Rice Genome Project) map (Harushima et al. 1998), the genetic distance between them is 15 cM. We are determining more precise location of ORL1 between S10628 and C377.


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Sato, Y., S.K. Hong, A. Tagiri, H. Kitano, N. Yamamoto, Y. Nagato and M. Matsuoka, 1996. A rice homeobox gene, OSH1, is expressed before organ differentiation in a specific region during early embryogenesis. Proc. Natl. Acad. Sci. USA 93: 8117-8122.

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